The present invention relates to an extractant for a rare earth metal and a method for extracting the same. Incidentally, the term "rare earth metal" is herein defined to encompass fifteen elements from lanthanum (atomic number: 57) to lutetium (atomic number: 71), scandium (atomic number: 21) and yttrium (atomic number: 39).
Since the rare earth metals have properties similar to one another, the rare earth metals covering from light rare metals such as lanthanum, cerium to middle and/or heavy rare earth metals such as lutetium are widely present, as a mixture, in, for instance, raw ores such as monazite and bastnaesite and thus they can only be separated into each element with great difficulties.
As methods for separating these rare earth metals into each individual element, there have presently been adopted, for instance, a method in which an ion-exchange resin is used and a method which makes use of extraction with a solvent.
Among these methods, the solvent extraction method comprises the step of bringing an acidic aqueous solution containing rare earth metals into contact with an organic solvent containing an extractant to thus selectively extract ions of a specific rare earth metal from the aqueous solution into the organic solvent. As the extractants conventionally used for such solvent extraction of ions of rare earth metals, there has been known, for instance, di-(2-ethylhexyl) phosphoric acid as disclosed in Japanese Unexamined Patent Publications (hereinafter referred to as "J. P. KOKAI") No. Sho 50-36316. This di-(2-ethylhexyl)phosphoric acid has a high extraction capacity against the ions of rare earth metals and is an excellent extractant for use in the separation and purification of the rare earth metals.
However, di-(2-ethylhexyl)phosphoric acid can maintain a high extraction capacity even in the low pH region and, therefore, the use thereof involves great difficulties in back-extracting the ions of rare earth metals extracted by di-(2-ethylhexyl)phosphoric acid into a water phase. For this reason, the use of a concentrated acid is required for the back extraction of the rare earth metal ions into the water phase. In particular, this tendency becomes more conspicuous when extracting heavy rare earth metals among others which have a high distribution factor for the phase of the extraction medium and they still remain in the phase of the extraction medium even if a large amount of an acid is employed for the back extraction. Therefore, they become a cause of contamination of the resulting product in the preparation of a highly pure rare earth metal and thus interfere with the high purification of the rare earth metals.
As a means for solving the problems associated with the foregoing di-(2-ethylhexyl)phosphoric acid, there have been proposed a variety of extractants which have a low extraction capacity and make the back extraction easy. Examples of such extractants include a branched chain alkylphosphoric acid ester represented by the following general formula: ##STR1## as disclosed in J.P. KOKAI No. Sho 54-112723 and mono 2-ethylhexyl 2-ethylhexylphosphonate as disclosed in J.P. KOKAI No. Sho 54-93672.
However, these compounds have a low distribution factor for the phase of the extraction medium with respect to the ions of the rare earth metals as compared with di-(2-ethylhexyl)phosphoric acid. Thus, these compounds make the back extraction easy, but they have an extraction rate lower than that for di-(2-ethylhexyl)phosphoric acid in the pH region at which the extraction of the rare earth metals is usually performed. For this reason, these compounds suffer from another problem in that the quality of these compounds as the extractant is inferior to that of di-(2-ethylhexyl)phosphoric acid (see Hydrometallurgy, 1989, 22, p. 121).